In computer science, a programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

 with a static type system is considered to be type sound if a well-typed program cannot cause type errors.
If an expression is assigned type t, and it evaluates to a value v, then v is in the set of values defined by t.
SML, OCAML, Scheme and Ada have sound type systems. Most implementations of C and C++ do not.